That is not a factory vacuum gauge for the '64-'66 Chevy truck gauge cluster. The factory vacuum or air gauge was available in the C30, C50 & C60 series truck dash gauge clusters.
This is a photo of the factory air gauge that is located in the left window of a factory dash gauge cluster in the '64-'66 Chevy trucks.
\'64-\'66 Chevy Truck Air Gauge There is a factory vacuum gauge, however I do not have a photo of one. The graphics and shape of the vacuum gauge are the same as the air gauge and fit in the left window, as well. I have seen them on eBay, in excess of $200.00.
The air gauge would be a good item to have if you have a bagged suspension. I have a couple, but never really had much of a use for them. Let me know if I can help.